Population is the study of the character, number, and distribution of living organisms residing in or migrating through particular places. The study of populations is a quantifiable foundation for concepts in sociology, ecology, genetics and evolution by means of natural selection. Study of population is closely associated with social and biological sciences and it examines the relative size of a breeding group with respect to the age structure, number of viable offspring, survival rates, and longevity among separate aggregations. Human demography, as a branch of sociology, is the study of the attributes of and changes in the aggregate number of people residing in particular communities around the world and their causes.

On August 1, 2010, the U.S. Census Bureau estimated the Earth's human population at over 6.859 billion people. It grew at an estimated rate of 1.133 percent in the year 2009. The United Nations Demographic Yearbook is an annual source of estimates for the population of its member countries. These international population estimates began with the work of the League of Nations in the 1920s. On August 1, 2010, the U.S. Census Bureau also estimated the United States population at nearly 310 million people.

As of February 2, 2013 the Earth's population was estimated to be 7,063,655,317 and the United States represented about 4.4 percent of the worlds' people or 315,259,511 population.

In this map the sizes of countries are proportional not to their actual landmass but instead to the number of people living there; a country with 20 million people, for instance, appears twice as large as a country with 10 million. (Source: University of Michigan)

Human population growth as an environmental issue

Because more than 81 million people per year are added to the human population, there persists a debate about the relative capacity of the Earth to sustain our current and projected population growth. Chief environmental factors negatively impacted by a human population beyond carrying capacity are biodiversity, ecosystem services and sustainable agriculture. The issue influences political decision making on national and international levels, because of the sorts of social programs directed to sustain women, children and families and the investments made by nations in promoting family planning, neo-natal (newly born children) care, and fertility control measures; moreover, matters of agricultural strategy and water resource use are strongly influenced by man's assessment of human population exceeding carrying capacity. Since gains in life expectancy as of the 1950s have occurred unevenly among nations, one driver of population growth since 1850 has been the increasing longevity among, especially affluent and peaceful, populations in the world. For example: in China, Honduras and Sweden the life expectancy, or age at which people in the population statistically may be anticipated to die, have all increased, while in Kenya it has declined since 1950.

Historical and constitutional contexts of population

This increase in the survival rates of young and old are a principle contributor to the growth of world population since World War One. The fall in the death rates and the subsequent decline in fertility rates have both lead to a surge in population. Since the eighteenth century, when the number of people in the United Kingdom's industrial areas began to grow, the causes and consequences of population growth have interested economists, sociologists, ecologists, policy makers, religious organizations and governments. Many governments, such as the United States, are constitutionally required to take a periodic census (or enumeration of the population) of the number of people in the nation to apportion the number of votes in the House of Representatives to reflect the loss or gain of people from one state or district within a state to another. Population growth and decline are viewed correctly as engines of both economic and environmental aspects of social change. However, there is no simple relationship between population growth rate and economic growth. It depends on a number of factors such as level of income, size and density of population, availabiltiiy of capital and age distribution of population.

Measuring population

An existing population fluctuates because of births, deaths and migrations. When determining the size of any population demographers consider the number born, or the crude fertility rate of an existing population and they subtract the number dying, or the crude mortality rate, to arrive at what is called the natural increase per year of any existing population. In addition to the rate of natural increase, net migration must be calculated to accurately reflect the changes in population. The net migrants are then accounted for by subtracting the number of emigrants--or people leaving an area-- from the number of immigrants--or the people coming into an area. That net migration figure is then added to or subtracted from the resident population to estimate the aggregate number of inhabitants in any place at a given time.

Fertility or natality

The number of births per one thousand people per year is called the annual crude birth rate and it is one of several measures of fertility in any population. Fertility rates in nations vary but crude birth rates of 43 in Chad or 46 per thousand in Angola are considered high rates, while 8 per thousand in Germany or Taiwan are considered low. The world average for the crude fertility rate is 11 per one thousand, but when less developed nations are averaged the figure is 20 per one thousand. A better measure of fertility is, however, total fertility rate (TFR). It is computed by adding age-specific fertility rates (number of births per woman) at all ages during the reproductive period, 15-45 years, or 15-49 years. A TFR of 2.1 indicates that the population has a long run tendency to stabilize. A TFR of more than 2.1 shows the growth potential of population. A TFR less than 2.1 shows that in the long run population may decline. Most of the developed countries have TFR less than 2. Several less developed populations have TFR close to 4.

Mortality

The number of deaths per one thousand people per year is called the annual crude death rate, or mortality. The relation between the age structure and the mortality rates in most populations coincide; the older the population, the higher the crude death rates. For example, 15 per thousand in Russia coincides with an aging population, as do 16 in the Ukraine. There are exceptions to this correlation of the crude death rates in an aging population. In the cases of Afghanistan and nations in Africa both war and infant mortality contribute to higher crude death rates among relatively younger populations. In Sierra Leone, war has raised the death rate to a high of 22 and 23 in Lesotho.

In other parts of Africa the HIV infection rate has had an effect on the mortality rates. In Swaziland, Zimbabwe, and South Africa where infection rates are high with one third to one fifth of the population estimated to have HIV, crude death rates are as follows: for Zimbabwe it is 23 per one thousand, South Africa 18 and Swaziland 28 per one thousand.

Experts and aid agency specialists have noted that poverty exacerbates the access to health care, adequate nutrition and clean water. In those poor nations the crude death rate is also higher than the world average which is 10 per one thousand of the population. For instance, in Africa where 66 percent of the population lives on less than $2 per day (equivalent in American dollars), the crude death rate averages 15 per one thousand. In Canada it is less that half that rate or 7 per thousand while in the United States rate rises to 8 and in Western Europe 9 per one thousand of the population.

Due to dependence of death rate on age distribution of population experts use life expectancy at birth as a better measure of mortality. It depends totally on age specific death rates and is independent of age distribution. Several countries in the world have life expectancy greater than 70 years. In most countries women have higher life expectancy than men.

Migration

The movement of people from one place to another is a fundamental characteristic of human and many herd animal populations. Those people leaving an area are called emigrants from the nation or place they leave and referred to as immigrants in the places they come to or in which they arrive. Net migration rates annually per one thousand vary widely from a high of 13 in Ireland to negative numbers in Poland, South Korea, Lithuania and the Netherlands. Lithuania has the fewest net migrant rate at a negative 2.6 in 2005. International migrants, it is estimated, comprise three percent of the world's population in any given period.

Population and carrying capacity

A rice terrace in Ubud, Bali. (Credit: Joseph Siry)

The number of living things in any area or place may vary but ecologists have suggested that there are several factors that limit the size of natural and even human numbers in any given situation. That limit, derived from the study of grazing animals in an acre or hectare of a field is called the carrying capacity. This capacity is derived from the number of individuals that the grazing land can nourish from one generation to the next without appreciable loss of food, water, and nutrients that sustain a herd of animals. For ecologists the population of organisms in any place or habitat cannot long exceed the carrying capacity of the arable land. The excess of the population above the carrying capacity triggers migration, or can lead to famine in human or starvation in animal populations.

Critics charge that ecologists cannot apply the findings from the study of herd animal populations to humans. Some economists and demographer's argue that rising populations improve economic conditions while declining populations, such as in the Great Plains communities of the United States and Canada foster economic decline. The debate remains contentious as ecologists insist that the capacity of the Earth to sustain more people and more affluent behavioral patterns is at, near or beyond its limits with often only trade, technology or other social policies able to extend the carrying capacity of wealthy regions. It may be noted that carrying capacity calculations would depend on the desired qualtiy of life: a country can afford to have more number of people if it is not to bother about high cultural standards available to people in the developed countries.